Aluminum alloys containing lithium are significantly less dense than conventional aluminum alloys and thus offer weight savings of around 10% for airplane structures in alloys containing between 2% and 21/2% of lithium. At present it is considered that aluminum alloys containing more than about 21/2% of lithium tends to weaken the alloy so that it would not be acceptable for us in commercial aircraft. Aluminum lithium alloys are now becoming available on a commercial basis and are being considered for use on current and future airplanes.
Aluminum ores are widely available, but the extraction process is energy-intensive and the cost of aluminum metal is largely dependent upon the expenses incurred during extraction. For this reason scrap aluminum alloys are of substantial economic importance, particularly in view of the fact that a large airplane manufacturer would have something of the order of 30 million pounds of aluminum scrap available for sale each year.
Normally, commercial aluminum scrap is recycled and converted back to usable form at a fraction of the cost of producing virgin metal. Primary producers generally reprocess their own scrap, but scrap produced by their customers is normally sold to secondary producers who process it into a variety of alloys, which are mainly used for casting purposes.
Aluminum-lithium alloys, however, present a special problem in that they are very reactive when molten, oxidize readily in air, and attack the refractories in furnace linings. It has, therefore, been postulated by the major primary producers that considerable difficulty could be encountered by secondary producers in attempting to reclaim aluminum lithium alloys by normal methods. They have also claimed that contamination of other alloys by small amounts of lithium could seriously affect their mechanical properties and render them unsuitable for die-casting processes.
The currently recommended procedure of one primary producer is to strictly segregate all aluminum-lithium scrap from other aluminum scrap in order to prevent cross-contamination. Furthermore, it is recommended that all aluminum-lithium scrap be recycled only by primary producers who need to be assured that it is not contaminated, and that if more than one aluminum-lithium alloy composition is used, they must be separated from each other. Such requirements are extremely costly in terms of man power, facilities, and transportation and could eliminate much scrap from the secondary market when aluminum-lithium becomes widely used.
If the advice of the primary producer or producers must be followed, the only alternative to recycling through the primary producers is to eliminate all aluminum-lithium material from the scrap loop by methods such as burial. This would still require segregation from other alloys and there would still be a risk of accidental contamination of the more conventional alloys by aluminum lithium alloys.
As indicated above, recycling of aerospace alloys into the secondary aluminum market is a multimillion dollar industry and elimination of such recycling would have a significant economic impact on aerospace production costs.
A search of the patent literature was made to attempt to find a method for the removal of lithium from aluminum-lithium alloys. No specific solution was found but U.S. Pat. No. 2,195,217 to Lindenburger et al. disclosed a method for reducing the metallic magnesium content of aluminum alloys. The invention in this patent was the discovery that slag forming materials such as alkali metal halides have the property of extracting magnesium from aluminum alloys. It was found to be important to provide for intimate contact between the slag-forming material and the aluminum-magnesium alloy while both were in a molten state. The process is carried out at a temperature above the melting point of both materials at temperatures of 1400.degree. F. to 1700.degree. F. or preferably from slightly above 1500.degree. F. to 1600.degree. F. An agitator arm, or other mixing device is used to insure intimate contact between the molten metal and the slag. In an example given, a charge of 10 lbs. of sodium chloride was placed in a crucible and melted and then 50 lbs. of an aluminum-magnesium alloy were added thereto. When the entire charge was melted, a temperature of 1450.degree.-1500.degree. F. being utilized, it was stirred vigorously for 30 minute. The metal was then drawn off from the slag and on analysis was found to contain less than 0.01% magnesium. For larger batches rotary furnaces were suggested. The order of the mixing of the metal and the slag was considered to be optional although it was preferred to provide a molten mass of the slag forming material and charge the metal thereto, applying heat, if necessary, to insure that the entire mass would remain molten for a length of time sufficient to effect the removal of the magnesium. The process was selective in that magnesium was removed whereas other metals such as copper, zinc, silicon, and maganese remained with the aluminum.
The following patents disclose processes for producing aluminum and lithium:
U.S. Pat. No. 710,493, Moeser et al. PA0 U.S. Pat. No. 1,180,435, Robison, C.S. PA0 U.S. Pat. No. 2,028,390, Hanson, M.G. PA0 U.S. Pat. No. 2,470,305, Gross, P. PA0 U.S. Pat. No. 2,621,120, Pedersen et al. PA0 U.S. Pat. No. 2,810,635, Cooper, H.S. PA0 U.S. Pat. No. 3,397,056, Layne et al. PA0 U.S. Pat. No. 3,856,511, Becker et al. PA0 Great Britain, No. 623,932, Phillip Gross PA0 Great Britain, No. 635,318, Ardel Verk
In addition to the foregoing, two basic methods are known in the aluminum industry for the removal of magnesium from molten aluminum alloys. The generally accepted method is to bubble chlorine gas through the molten alloy until sufficient magnesium has been removed as magnesium chloride. A less commonly used method is to cover a molten bath with a flux of metal fluoride. This method is less noxious and poses fewer safety problems, but is much slower than the chlorine process. Although the second method utilizes nonnoxious chemicals, resultant particulate emissions are considered hazardous waste. Aluminum smelting facilities use standard "baghouses" that readily control these emissions.
It is known that lithium behaves similarly to magnesium and would react with chlorine or fluorine. Both of the above methods could be used for lithium extraction but because aluminum-lithium alloys oxidize readily in air and attack the refractory furnace linings, these methods are not practical for normal extraction operations, because they require mixing the flux into a molten metal bath or bubbling reactants through the bath. The methods are also time consuming. In addition, the large primary producers of aluminum-lithium alloys have had to use special refractories to keep reactions to a minimum.